For me, the secondary characters seem to initially appear like paper characters in a pop-up book. They are exactly that, cardboard figures whose purpose is known but they're not fully fleshed out. Eventually, I know I need them for tasks large and small. Especially the sidekick. I can't imagine not having one for either my hero or heroine or both.
I think of a sidekick in dueling terms; the hero's second. A true friend. Faithful and loyal, the second has the hero's back. Or the heroine's. Heroines need seconds too. The sidekick might also be comic relief; a needed contrast to a brooding hero. The sidekick can also shine a light on the hero or heroine. Friendship says a lot. It means validation. The hero or heroine must be a true friend to have a true friend. And the love interest of the hero or heroine might need to prove to the sidekick how worthy he or she is of the hero or heroine. This makes for an excellent source of conflict. The sidekick's opinion says a lot that the hero or heroine's own introspection and dialogue might not accomplish.
In my current WIP, I find the sister of the hero a helpful sidekick. Who knows the hero better than his sister? He's the big brother who slayed dragons for her. By showing her belief that the heroine is worthy of her brother's love, I hope it gives a boost to the heroine.
Beyond the characterization, she really come in handy for the plot. Heroine needs to be broken out of a mental hospital? Bring in the sidekick. Need a computer expert? Bring in the sidekick. Yes, she's a useful character to have around! When I delve deeper, the sidekick has a huge purpose.
So does your hero or heroine have a sidekick?
P.S. Announcements coming up! Not one but two!